In code-division multiple access (CDMA) communication systems, communication between a base-station and a subscriber unit is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code. As a result, transmitted signals are in the same frequency band of the communication channel and are separated only by the unique user spreading codes. These unique user spreading codes preferably are orthogonal to one another such that the cross-correlation between the spreading codes is substantially zero.
Particular transmitted signals can be retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a user spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. Further, when the user spreading codes are orthogonal to one another, the received signal can be correlated with a particular user spreading code such that only the desired user signal related to the particular spreading code is enhanced while the other signals for all of the other users are significantly de-emphasized.
To maintain orthogonality among all channels, the channels must be substantially time aligned on a per-chip basis. For forward channel transmission (from base-station to subscriber unit), this time alignment is easily obtained via a synchronized process of combining the traffic channels together at each basestation. However, time alignment of reverse channel signals (from subscriber units to a particular base-station) is not as simple due to the time-randomness of transmissions by subscriber units. Attempts have been made to synchronize the reverse channel signals at a base-station, but such attempts require extra circuitry at the base-station, which in turn leads to both increased complexity and added cost to the base-station.
Thus, a need exists for a simple and cost effective method and apparatus for time aligning signals for reception in a CDMA communication system.